The present invention concerns a drilling tool incorporating blades fitted with secondary cutting edges and a system making it possible to remove the material excavated by these secondary cutting edges.
The problem which inspired the invention will be explained with reference to FIGS. 1 and 2, which show, respectively, a perspective view of a drilling tool incorporating conventional blades and secondary cutting edges, and an enlarged transverse cross-section along line II--II in FIG 1.
The cutting tool 10 illustrated in FIG. 1 comprises a head 12 fitted with a tubular threaded connector, which is used to mount the tool on a drive tube train (not shown). The tool may be driven in rotation around its axis with a slight downward helicoidal motion, in the direction of the arrow f. Multiple projecting ribs, or blades, 16 separated by grooves 18 are formed around the head. Each blade has an outer face 20 and two lateral faces 22, 24 which extend downward toward the bottom of the grooves bordering the blade.
The driving edge 26 of each blade, that is, the edge which first encounters the deposit, has embedded along it a series of primary cutting edges 28 of any conventional type, for example cutting edges formed as inserts made of natural or synthetic diamond or of PDC, which are fastened to the blade.
Back-up, or secondary, cutting edges 40 positioned in at least the cutting face area of the tool are fastened to the upper face of the blades and to the rear of the primary cutting edges, as determined by the direction of travel of the blade.
When in operation, the primary cutting edges break up the deposit, and the resulting excavated material is carried away along he grooves 18. But, because of the helicoidal movement of the tool inside the drill hole, the secondary cutting edges may come in contact with areas of the deposits which have not previously been broken up by the primary cutting edges. The secondary cutting edges then produce debris which, because no path for removal exists, accumulates in the spaces 29 between the second and primary cutting edges. The result is the localized packing of material and a significant loss of tool efficiency.
This packing tendency is exacerbated when the tool is used to drill soft deposits or those exhibiting a high degree of plasticity, since these deposits tend to warp plastically to the inside of the drill hole after the primary cutting edges have passed. The secondary cutting edges dig into the deformed areas and, here again, produce excavated material which accumulates in the spaces 29.
Until now, to avoid packing in the spaces 29, normal practice entailed dispersing the excavated material using jets of liquid that are emitted under high pressure through nozzles positioned in said spaces. However, in practice, the use of this process to carry away the material gives mediocre results because of the narrowness of the spaces.